1. Field of the Invention
The present invention relates to an optical device for analyzing a scattering medium held by a support.
2. Discussion of the Related Art
A subject area of optical imaging relates to the analysis of a scattering medium, for example, the analysis of biological tissues.
Generally, the scattering medium is submitted to an incident light beam. For certain applications, for example, in optical diffusion tomography, the wavelength of the incident beam is in the visible spectrum, conventionally in the red or in the near infrared, which corresponds to the wavelength range where the absorption of the biological tissues is minimum. The incident light beam scatters into the scattering medium and the scattered light radiation which escapes from the scattering medium is detected by a sensor, for example, a camera. The images provided by the sensor are analyzed to determined the optical properties of the scattering medium, for example, to determine the distribution of the optical diffusion and/or absorption properties of the scattering medium, for example, to detect the presence of an abnormally absorbing and/or scattering area.
For other applications, for example, in fluorescence optical diffusion tomography, specific fluorescent tags which tend to concentrate in areas of interest are arranged in the scattering medium. When they are illuminated by the incident light beam, the tags emit a radiation at a wavelength distinct from that of the incident beam. This radiation scatters into the scattering medium and is detected and analyzed to determine the properties of the tags (for example, the location and the local concentration of the tags).
In the field of medical imaging, optical imaging techniques provide, for example, to detect and locate carcinomae, an alternative to conventional imaging techniques such as radiography and X-ray laminography, positron emission tomography, magnetic resonance imaging, etc.
For certain optical devices, the light source is in direct contact with the scattering medium. As an example, the incident light beam is guided all the way to the scattering medium by optical fibers. The scattered radiation emitted by the scattering medium may also be collected by optical fibers and guided all the way to the sensor. An advantage is that the environment of the scattering medium, in particular the support holding the scattering medium, does not disturb the measurements. However, such optical devices are generally bulky since the accessible accuracy of the distribution of the optical properties depends on the number of independent measurements that may be performed, that is, on the number of optical fibers used.
For other optical devices, the light source is distant from the scattering medium. The structure of the optical device can then be simplified. The support holding the scattering medium is generally designed to disturb as little as possible the propagation of light rays. For this purpose, the support is formed of a material non-absorbing and non-scattering for the useful wavelength range.
FIGS. 1 and 2 show two embodiments of optical devices for which the light source is distant from the scattering medium. Each optical device 10, 12 comprises a light source 14, for example, a laser diode providing a monochromatic light beam 15 having a wavelength of, for example, 670 nanometers, and an image sensor 16 adapted to the detection of a scattered radiation. An analysis tool 17 is connected to sensor 16 and processes the images acquired by sensor 16. A scattering medium to be analyzed 18, for example, a mouse, is arranged to receive incident light beam 15 provided by light source 14. A lens 19 is arranged between scattering medium 18 and sensor 16. It is possible for the axis of sensor 16 not to be parallel to incident beam 15. According to a variation, the images provided by sensor 16 may be recorded by tool 17 and subsequently analyzed by means of another computer.
For device 10 shown in FIG. 1, medium 18 to be analyzed is held by a support 20 comprising an input plate 22 located between medium 18 and light source 14 and an output plate 24 located between medium 18 and sensor 16. Plates 22, 24 are transparent and parallel. As an example, support 20 may be fixed and light source 14 may be mobile with respect to support 20. For device 12 shown in FIG. 2, medium 18 to be analyzed is held by a support 30 corresponding to a transparent tube comprising a cylindrical lateral wall 32 and a planar bottom 34. As an example, support 20 is mounted on a rotating arm 38, while source 14 may be fixed.
Support 20, 30 is generally made of plastic or glass. However, the use of such supports may result in the forming of artifacts on the images provided by sensor 16. Such artifacts may disturb, or even hinder the analysis of scattering medium 18.